Useful rubber products, such as automotive tires, are usually molded from vulcanized rubber. In general, vulcanization is the chemical treatment of a hydrocarbon rubber polymer such that the long polymer chain molecules cross-link with molecules of a cross-linking agent. The reaction transforms the soft, weak, plastic polymeric material into relatively strong, elastic material resistant to deterioration and permanent deformation. One of the most common cross-linking agents is sulfur. When sulfur or sulfur donors are used as the cross-linking agent, sulfur or sulfur donors and an accelerator or several accelerators, such as a primary and a secondary accelerator are mixed with the rubber polymer. The mixture is subjected to pressure and heat, resulting in the polymer chain molecules cross-linking with sulfur molecules.
The addition of sulfur provides beneficial physical properties of a final rubber product, such as strength, elasticity, and durability. Unfortunately, the benefits achieved present a problem when attempting to recycle vulcanized rubber. The cross-linked rubber polymer cannot be reclaimed into a useful product merely by heating and reprocessing. The strength of the cross-linked bonds prevents the vulcanized rubber from melting, dissolving, or binding with a virgin rubber matrix. This has, in part, facilitated the creation of a scrap tire problem in the United States.
Vulcanized rubber is the source of one of the most serious waste product problems in the United States. Millions of tons of natural and synthetic rubbers are produced and consumed every year. This volume of production yields a substantial amount of scrap rubber, including used tires. Scrap tires constitute a large percentage of the total scrap rubber stockpiled and discarded annually. For example, billions of tires have been discarded with approximately 273 million tires discarded annually. Yet, only a fraction of waste tires generated annually is truly recycled, most are burned as fuel.
The enormous quantity of scrap rubber has created a serious disposal and environmental problem. In addition to disposal concerns, waste tires pose serious fire and safety hazards. The problems and concerns continue to mount as an increasing number of landfills refuse to accept scrap rubber waste. Approximately a dozen states have banned all scrap tires from their landfills, and about twice that number, allow only processed tires in the landfills.
The demand for satisfactory recycling processes is on the rise as the availability of landfills diminishes. A number of approaches have been employed to try to solve the waste rubber problem. One method involves shredding and grinding tires into small crumbs and then binding the rubber with polyurethane into low-performance products such as rubber mats, gaskets, and vehicle mud flaps. Crumb rubber has been used as an additive in road asphalt, as ground coverings for surfaces such as parking lots and playgrounds, and as fill under road surfaces or turf such as athletic fields and golf courses. In addition, scrap rubber has been burned as fuel and subjected to pyrolysis to produce oil that is used as a fuel. However, these approaches generally produce a low quality rubber product, or present additional pollution problems.
When using crumb rubber to form new products, the crumb may be used as filler for mixing with virgin rubber. Generally, however, only low percentages of crumb filler may be added before the properties of the compound begin to degrade. The major drawback of conventional crumb to is that the cross-link bonds created by the vulcanization process are very strong. This results in rubber compound molecules having a “memory” effect. The memory effect enables rubber items to regain their original shape after being subjected to a deforming force. For example, a rubber band that is stretched returns to its original shape. Even when vulcanized rubber is processed into crumbs, the rubber molecules retain much of their physical properties. The strength of the cross-link bonds makes binding of the vulcanized rubber within a virgin rubber matrix difficult—the bonding between the new rubber and the vulcanized crumb is weak. Thus, only about 3 to 4 percent by weight of reclaimed rubber can be used in the manufacture of new tires.
More crumb may be added as filler if the surface of the crumb is modified to enhance the ability of the crumb to be incorporated into the mix. For example, the rubber molecule chains on the surface of the crumb may be chemically modified or the surface may be coated to help the rubber crumb blend into the mix. However, the process does not change the inert nature of the crumb core, and the amount of surface-modified crumb that can be added as filler is still limited.
Another approach to recycling rubber waste is devulcanization of crumb rubber. Devulcanization is intended to depolymerize rubber molecules or break the polysulfide linkages without removing the combined sulfur. Methods for devulcanization may include mechanical shear, high-energy radiation, chemical processing, and thermo-mechanical processing. Although vulcanization is not completely reversible, partial devulcanization results in a decreased resistance to deformation and allows an increased amount of devulcanized crumb to be added to fresh rubber mixes than can be added using unmodified or untreated crumb. Devulcanized or reclaimed rubber can be revulcanized with or without the addition of natural or synthetic rubbers or binders.
Current devulcanization or reclaiming methods have several drawbacks. For example, current methods may use high temperatures (e.g., 150.degree. C. to 250.degree. C.) to digest the elastomeric material being recycled, and may require stirring for many hours (e.g., 5 to 12 hours). This results in a high consumption of energy and degradation of the material being reclaimed. For example, the intense heat and mechanical shear actions of a thermo-mechanical process may cause some polymeric chain backbone to break down. In addition, because this process does not use chemicals, it is not possible to selectively cleave targeted polysulfidic bonds. Some processes, such as solvent extraction, utilize large quantities of chemicals or solvents that can damage the environment, or involve complicated solvent or chemical recovery and treatment. Still other reclamation processes, such as microwave or ultrasonic processing, employ relatively complicated equipment that is difficult to obtain and difficult to operate efficiently for industrial production.
Rubber reclaimed using such approaches tends to have poor physical properties. For example, conventionally reclaimed rubber may have tensile strength of about 5 to 6 MPa, while natural rubber may have tensile strength over 20 MPa.
For some products that do not require high strength, such as rubber mats and road markers, larger percentages of vulcanized rubber crumb from scrap tires may be used. However, the presence of recycled rubber, either treated or untreated, in a mixture with fresh rubber compounds, may adversely affect the physical properties of the final product. As a result, reclaimed rubber is typically not used in the manufacture of automobile tires.
Therefore, a need still exists for a method of devulcanization that permits a greater portion of vulcanized scrap rubber to be re-used, while still meeting required specifications of the finished product.